1. Field of the Invention
The present invention relates to a probe for an ultrasonic gauge, and in particular, to a probe having a delay line construction that enables probe measurements to be adjusted to compensate for changes in temperature of the delay line.
2. Discussion of Related Art
Ultrasound provides an ideal physical mechanism to investigate surface flaws or the thickness of coatings on substrata with acoustically different properties. When a coating is applied to a substrate that has acoustic properties that are different from those of the coating, an acoustic coating/substrate interface is created. At such an interface or at a flaw, an ultrasonic vibration will be partially reflected.
For example, an ultrasonic vibration, also known as an impulse, can be transmitted into a coating using a resonant piezo element transducer. The same transducer can also be setup to "listen" for echoes created when the impulse reflects from the interface of the coating and substrate back to the transducer. The output of the transducer can be recorded for a known period after the impulse has been transmitted. This period is defined as an echo window. The echo window is defined to overlap with the time of expected echoes of interest.
By analyzing the echo recorded during the echo window, it is possible to determine the location of a flaw or an interface between the coating and the substrate. The thickness of the coating can be determined if the velocity of sound within the coating material and the time of the interface echo are known. In other words, the thickness of the coating can be determined by multiplying the velocity of the vibration through the coating material times the time for the vibration to enter the coating, reflect off the interface, and exit the coating, and dividing that product by two. EQU Thickness=(Velocity.times.Time)/2
The resolution of the derived thickness is limited by the temporal resolution of the sampled echo. Improvements in the resolution of the sampled echo will directly improve the resolution of the derived thickness.
In U.S. patent application, Ser. No. 08/127,529, a detailed description of an improved ultrasonic gauge is disclosed. The subject matter of U.S. patent application, Ser. No. 08/127,529, is hereby incorporated herein by reference.
When using ultrasound to investigate near surface flaws, such as in the case with thin coatings, a delay line or stand off block is frequently required to separate the noise associated with the transducer excitation signal. Thus the practical investigation of near surface flaws requires the use of such an acoustic delay line.
The determination of the reflection times for closely spaced flaws represents a difficulty when the flaw or coating/substrate interfaces are less than one wavelength of the frequency of investigation. To overcome this limitation, several techniques can be employed, such as increasing the frequency of the ultrasonic impulse or using relatively low frequencies coupled with the use of matched delay lines.
With a high frequency broad band transducer, it is possible to separate surface echoes from flaw echoes. A 100 MHz ultrasonic signal has a wavelength of approximately 10.6 .mu.m in polystyrene, whereas a 10 MHz signal has a wave length of 106 .mu.m. Clearly the 100 MHz signal is able to resolve finer detail.
The use of a delay line matched to the coating material under investigation allows the use of lower frequencies by eliminating the need to separate the surface entry echo from the echo from a flaw or the far side of the coating. It is advantageous to use lower frequencies for several reasons, among them being lower instrumentation costs and complexity as a result of the reduced bandwidth requirements. In addition, the transducer construction is made considerably easier.
When constructing matched delay lines the acoustic properties of the delay element may be less than ideal since material characteristics are imposed by the coating characteristics under investigation. Consider a thin polymer based coating applied to a dissimilar substrate. Since the coating is a polymer, the delay line must also be of a similar material. The use of a matched delay line requires that the delay tip reflection be known so that it can be distinguished from the flaw reflection to determine thickness or flaw location. Typically the delay tip reflection can be obtained by removing the transducer from the test piece and measuring the time for a vibration to travel from the transducer to the delay line material/air interface, and back to the transducer again. This propagation time is retained for future use and is commonly referred to as the probe "zero". When the delay line material is coupled to the test piece, a measurement of the time for a vibration to travel from the transducer to the far side of the test piece, and back to the transducer again is obtained. When the probe "zero" time is subtracted from the time obtained from the test piece, the thickness of the test piece can be readily calculated.
One difficulty with such a technique is that the probe "zero" time can change with variations in temperature due to the velocity changes in the delay material. Furthermore, delay lines of typical construction are relatively long compared to the coating thickness. As a result the errors associated with the temperature related velocity change in probe "zero" temporal location are significant.
When measuring very thin coatings, small changes in temperature, such as those brought about by simply touching the transducer can cause measurement errors on the order of 100% of the coating thickness itself.
One prior art method of compensating for such temperature changes is disclosed in U.S. Pat. No. 4,437,332, issued to Pittaro. Pittaro discloses a method of calibrating an instrument by measuring the time required for a vibration to travel round trip to and from a calibration specimen of a known thickness. The system then computes an offset time value to compensate the system for changes in the velocity of propagation. The offset time value is then used to measure objects of unknown dimension. However, the system can only be calibrated by applying the delay line to a calibration specimen of a known thickness. Thus, calibrating the system is time consuming and cannot easily be done before each measurement. As a result, the delay line may experience changes in the temperature, as a result of handling or other causes, between calibrations that will negatively affect measurements.
The delay line disclosed by Pittaro also includes a notch, wherein the time for a signal to travel back and forth between a transducer and the notch can be used to identify the delay line. However, inaccurate identifications may occur if the delay line experiences temperature changes that affect the velocity of propagation of the signal in the delay line.